We study the depinning of domain walls by pure diffusive spin currents in a nonlocal spin valve structure based on two ferromagnetic Permalloy elements with copper as the nonmagnetic spin conduit. The injected spin current is absorbed by the second Permalloy structure with a domain wall, and from the dependence of the wall depinning field on the spin current density we find an efficiency of 6×10{-14} T/(A/m{2}), which is more than an order of magnitude larger than for conventional current induced domain-wall motion. Theoretically we find that this high efficiency arises from the surface torques exerted by the absorbed spin current that lead to efficient depinning.
High-purity, type IIa diamond is investigated by noncontact atomic force microscopy ͑NC-AFM͒. We present atomic-resolution images of both the electrically conducting hydrogen-terminated C͑100͒-͑2 ϫ 1͒ :H surface and the insulating C͑100͒-͑2 ϫ 1͒ surface. For the hydrogen-terminated surface, a nearly square unit cell is imaged. In contrast to previous scanning tunneling microscopy experiments, NC-AFM imaging allows both hydrogen atoms within the unit cell to be resolved individually, indicating a symmetric dimer alignment. Upon removing the surface hydrogen, the diamond sample becomes insulating. We present atomic-resolution images, revealing individual C-C dimers. Our results provide real-space experimental evidence for a ͑2 ϫ 1͒ dimer reconstruction of the truly insulating C͑100͒ surface. Hydrogenated and clean diamond surfaces ͓see model in Figs. 1͑a͒ and 1͑b͔͒ have attracted considerable interest in recent years, motivated by the unique electronic, thermal, mechanical, and optical properties of diamond, 1-3 which makes it suitable for high power laser and gyrotron applications or field-effect transistors. 4 Most of the diamond samples, both natural diamond and artificial chemical-vapor deposition ͑CVD͒ diamond, have a low impurity concentration and are insulating. For example, so-called type IIa diamond exhibits an impurity concentration of less than 1 ppm. Therefore, type IIa diamond is not sufficiently conducting for scanning tunneling microscopy ͑STM͒ imaging unless a water layer is present. 5,6 Thus, atomic force microscopy ͑AFM͒ appears to be the ideal tool for a high-resolution study of the diamond surface. Highest resolution has recently been demonstrated for bare dielectric surfaces 7-9 and molecules on insulators, 10-13 including submolecular resolution of a pentacene molecule at low temperature. 14 However, despite many attempts, atomic-scale AFM imaging of diamond surfaces has not been successful so far. To progress further in our understanding of diamond, a detailed characterization of its surface structure is necessary.In this Rapid Communication, we present atomically resolved noncontact AFM ͑NC-AFM͒ images that reveal the individual hydrogen atoms on the hydrogenated diamond ͑100͒ surface, and the C-C dimers on the hydrogen-free diamond ͑100͒ surface. This is in contrast to high-resolution STM images of the hydrogenated diamond 6,15-18 and hydrogen-free diamond surfaces.19-21 STM images of the hydrogenated diamond ͑100͒ surface could not separate the hydrogen atoms, which are clearly resolved in the NC-AFM image presented here. On the clean diamond ͑100͒ surface, individual C-C dimers could not be seen with the STM whereas they are clearly visible with the NC-AFM as demonstrated in this work. As a matter of fact, NC-AFM offers greatly enhanced resolution compared to STM for both the hydrogenated and clean diamond surfaces.
In a non-contact atomic force microscope, based on interferometric cantilever displacement detection, the optical return loss of the system is tunable via the distance between the fiber end and the cantilever. We utilize this for tuning the interferometer from a predominant Michelson to a predominant Fabry-Pérot characteristics and introduce the Fabry-Pérot enhancement factor as a quantitative measure for multibeam interference in the cavity. This experimentally easily accessible and adjustable parameter provides a control of the opto-mechanical interaction between the cavity light field and the cantilever. The quantitative assessment of the light pressure acting on the cantilever oscillating in the cavity via the frequency shift allows an in-situ measurement of the cantilever stiffness with remarkable precision.
We have investigated the magnetoresistance of Permalloy (Ni80Fe20) films with thicknesses ranging from a single monolayer to 12 nm, grown on Al2O3, MgO and SiO2 substrates. Growth and transport measurements were carried out under cryogenic conditions in UHV. Applying in-plane magnetic vector fields up to 100 mT, the magnetotransport properties are ascertained during growth. With increasing thickness the films exhibit a gradual transition from tunneling magnetoresistance to anisotropic magnetoresistance. This corresponds to the evolution of the film structure from separated small islands to a network of interconnected grains as well as the transition from superparamagnetic to ferromagnetic behavior of the film. Using an analysis based on a theoretical model of the island growth, we find that the observed evolution of the magnetoresistance in the tunneling regime originates from the changes in the island size distribution during growth. Depending on the substrate material, significant differences in the magnetoresistance response in the transition regime between tunneling magnetoresistance and anisotropic magnetoresistance were found. We attribute this to an increasingly pronounced island growth and slower percolation process of Permalloy when comparing growth on SiO2, MgO and Al2O3 substrates. The different growth characteristics result in a markedly earlier onset of both tunneling magnetoresistance and anisotropic magnetoresistance for SiO2. For Al2O3 in particular the growth mode results in a structure of the film containing two different contributions to the ferromagnetism which lead to two distinct coercive fields in the high thickness regime.
SummaryInterferometric displacement detection in a cantilever-based non-contact atomic force microscope (NC-AFM) operated in ultra-high vacuum is demonstrated for the Michelson and Fabry–Pérot modes of operation. Each mode is addressed by appropriately adjusting the distance between the fiber end delivering and collecting light and a highly reflective micro-cantilever, both together forming the interferometric cavity. For a precise measurement of the cantilever displacement, the relative positioning of fiber and cantilever is of critical importance. We describe a systematic approach for accurate alignment as well as the implications of deficient fiber–cantilever configurations. In the Fabry–Pérot regime, the displacement noise spectral density strongly decreases with decreasing distance between the fiber-end and the cantilever, yielding a noise floor of 24 fm/Hz0.5 under optimum conditions.
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